Phenol is an important product in the chemical industry and is useful in, for example, the production of phenolic resins, bisphenol A, ε-caprolactam, adipic acid, and plasticizers.
Currently, the most common route for the production of phenol is the Hock process via cumene. This is a three-step process in which the first step involves alkylation of benzene with propylene in the presence of an acidic catalyst to produce cumene. The second step is oxidation, preferably aerobic oxidation, of the cumene to the corresponding cumene hydroperoxide. The third step is the cleavage of the cumene hydroperoxide in the presence of heterogeneous or homogeneous catalysts into equimolar amounts of phenol and acetone, a co-product. However, the world demand for phenol is growing more rapidly than that for the acetone co-product. In addition, due to developing shortages in supply, the cost of propylene is likely to increase.
Thus, a process that avoids, or reduces the use of propylene as a feed and coproduces higher ketones, rather than acetone, may be an attractive alternative route to the production of phenol. For example, there is a growing market for cyclohexanone, which is used as an industrial solvent, as an activator in oxidation reactions and in the production of adipic acid, cyclohexanone resins, cyclohexanone oxime, caprolactam, and nylon 6.
It is known that phenol and cyclohexanone can be co-produced by a variation of the Hock process in which cyclohexylbenzene is oxidized to obtain cyclohexylbenzene hydroperoxide and the hydroperoxide is decomposed in the presence of an acid catalyst to the desired phenol and cyclohexanone. Although various methods are available for the production of cyclohexylbenzene, a preferred route is disclosed in U.S. Pat. No. 6,037,513, which discloses that cyclohexylbenzene can be produced by contacting benzene with hydrogen in the presence of a bifunctional catalyst comprising a molecular sieve of the MCM-22 family and at least one hydrogenation metal selected from palladium, ruthenium, nickel, cobalt, and mixtures thereof. The '513 patent also discloses that the resultant cyclohexylbenzene can be oxidized to the corresponding hydroperoxide which is then decomposed to the desired phenol and cyclohexanone co-product in roughly equimolar amounts.
There are, however, a number of problems associated with producing phenol via cyclohexylbenzene rather than the cumene-based Hock process. One such problem is that the cyclohexanone and phenol produce an azeotropic mixture composed of 28 wt % cyclohexanone and 72 wt % phenol. Thus, while some high purity cyclohexanone can be recovered from the product of the '513 patent by simple distillation, production of high purity phenol requires a different separation approach.
One convenient approach is by extractive distillation. This method uses a solvent, which desirably has a lower volatility than the lowest volatility of the component in the mixture to be separated, is miscible with the mixture and the components therein, and does not form an azeotrope with the mixture or any of its components. Conveniently, the solvent interacts differently with the components of the azeotropic mixture thereby causing their relative volatilities to change. This enables the new three-part system to be separated in a simple distillation device or devices. The original component with the greatest volatility separates out as the top product, while the bottom product comprises the solvent and the lower volatility component. This bottoms product can again be separated easily because the solvent doesn't form an azeotrope with the lower volatility component.
Various solvents have been proposed for the separation of azeotropic phenol systems over the years. For example, for the phenol-cyclohexanone system, U.S. Pat. No. 2,265,939 discusses the use of diols and glycols as a solvent. This patent notes that ethylene glycol will react with the cyclohexanone to form ketals which co-distill with cyclohexanone, and recovery of the reacted cyclohexanone and ethylene glycol must be effected by conducting a hydrolysis reaction. It further notes that to avoid the reaction of cyclohexanone and the solvent, larger molecules providing a greater atomic distance between the two hydroxyl groups of a diol or glycol, such as diethylene glycol, should be employed.
U.S. Pat. No. 5,334,774 discusses the use of diethylene glycol to effect separation between the azeotropic system of phenol and sec-butylbenzene.
In U.S. Pat. No. 4,230,638, mixtures of sulfolane, diethylene glycol, and non-oxygenated hydrocarbons are proposed as solvents in a liquid-liquid extraction system for the separation of cyclohexylbenzene from phenol and cyclohexanone. However, sulfolane, while having outstanding solvent qualities for this separation, is not preferred due to its high reactivity with oxygen. Air ingress is difficult to avoid in any distillation process conducted at vacuum pressures and, with sulfolane as a solvent, can result in the production of acids and other deleterious degradation products. Diols and glycols tend to be preferred as they are far more resistant to undesirable side-reactions with oxygen.
According to the present disclosure, it has now been found that diols and glycols having their hydroxyl groups attached to non-adjacent carbon atoms can undergo a reaction with cyclohexanone to form a previously undocumented class of hemiketal and enol-ether condensation products under certain conditions. Specifically, we have uncovered the formation of large acyclic hemiketals, and also their cyclic olefin/ether water elimination products (enol-ethers) under certain conditions, which can affect the separation of phenol and cyclohexanone. Recognition of this fact, and the characteristics of these new compounds, is important to the proper design and operation of extractive distillation systems for the separation of phenol and cyclohexanone using such larger diols and glycols as solvents in general, and diethylene glycol in particular.